[WIP] Add configurable startup script execution before UI initialization

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

• Add a configurable startup_script (a Python function) that runs before the application UI is fully initialized
• Show a visual indicator (like a splash screen or loading dialog) while this script executes
• Have this happen before any stackers are initialized

Custom agent used: developer-agent
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